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Development of a Lesion-Mimic Phenotype in a Transgenic Wheat Line Overexpressing Genes for Pathogenesis-Related (PR) Proteins Is Dependent on Salicylic Acid Concentration

In the course of coexpressing genes for pathogenesis-related (PR) proteins for a class IV chitinase and an acidic glucanase in transgenic wheat plants, we regenerated a wheat line that developed necrotic lesions containing dead cells in the T₂ and subsequent generations. Lesion spots were detected at the booting stage (5- to 6-week-old plants) in lines homozygous for the transgene loci. In contrast, lesions were not observed in hemizygous transgenic lines or lines silenced for transgene expression, indicating a requirement for high levels of transgene expression for the development of the lesioned phenotype. Lesion development was associated with the accumulation of host-encoded PR proteins, e.g., chitinases, β-1,3-glucanases, thaumatin-like protein, and production of reactive oxygen intermediates. F₁ progeny of a cross between the lesion-plus transgenic line and wild-type nontransgenic plants produced progeny with a normal phenotype, while the F₂ progenies segregated for the lesion phenotype. Salicylic acid (SA) levels in plants with the lesion-plus phenotype were found to be several times higher than controls and nearly double the levels in hemizygous transgenic plants that lack lesions. SA application activated lesion development in excised leaf pieces of these hemizygous transgenic plants. Similar activation of lesion development in control plants occurred only when high concentrations of SA were applied for prolonged periods. Transcripts for phenylalanine-ammonia lyase, which provides precursors of SA, were elevated in homozygous transgenic plants. Our data suggest that transgene- induced lesion-mimic phenotype correlates with enhanced SA biosynthesis.